The present invention relates to an apparatus for open-loop or closed-loop hydraulic flow control in a hydraulic system with a variable-speed pump which is connected on the intake side to a reservoir and on the delivery side via a throttle to a consumer system, for example a power-assisted steering system of a motor vehicle. A flow control piston controls a connecting path between the intake side and the delivery side of the pump and is actuated by the pressure of the delivery side into a position releasing the connecting path and by a restoring spring and the pressure downstream of the throttle into a position shutting off the connecting path. A pressure relief valve arranged on the delivery side of the pump releases with a throttling effect a connecting line leading to the intake side if a pressure threshold value is exceeded.
In the case of motor vehicle hydraulic systems, the associated hydraulic pumps are usually driven directly by the vehicle engine, so that the delivery member of the pump is driven with greatly varying speed, corresponding to the greatly varying speeds of the vehicle engine during a driving operation. In such cases, the pump is generally dimensioned in such a way that the volume flow necessary for operation of the respective hydraulic system is already generated by the idling speed of the vehicle engine. In other words, at relatively high speeds of the vehicle engine there may be a delivery capacity available which far exceeds what is required. The volume flow passed to the consumer system is then limited to the respective requirement by the flow control piston.
An apparatus of the aforementioned type for open-loop or closed-loop hydraulic flow control is described in Swiss Patent Specification 362,613 in connection with a motor vehicle power-assisted steering system. In normal operation of the power-assisted steering system, hydraulic medium constantly flows from the delivery side of the pump via the servo valve to a hydraulic reservoir. Consequently, a pressure drop occurs at the throttle connecting the servo valve to the delivery side of the pump, with the throttle being arranged and designed as part of the flow control piston. The pressure drop has the effect that a force in the opening direction is exerted on the flow control piston. Consequently, in normal operating states of the servo valve, the flow control piston can adapt the supply of hydraulic medium to the servo valve according to requirements. Depending on the requirements of the servo valve and the delivery capacity of the servo pump, a greater or lesser proportion of the hydraulic medium delivered by the pump is introduced into the connecting path to the intake side of the pump.
In extreme operating states of the servo valve, for example if a great resistance in the vehicle steering has to be overcome, no hydraulic medium can flow off via the servo valve to the hydraulic reservoir. Consequently, the pressure on the delivery side of the pump increases until the pressure relief valve opens, which in the known arrangement controls a connecting line, branching off from the delivery line of the pump upstream of the connecting path controlled by the flow control piston, to the intake side of the pump.
In particular, when the vehicle engine is running at high speed and the pump is correspondingly generating a very strong delivery flow, great quantities of the hydraulic medium flow through the pressure relief valve. For as long as the servo valve assumes an extreme operating state in which no hydraulic medium can flow via the servo valve to the hydraulic reservoir, the connecting path, controlled by the flow control piston, to the intake side of the pump also remains closed because no pressure drop can occur at the throttle leading to the servo valve. Correspondingly, no hydraulic forces can be generated which would urge the flow control piston into a position opening the connecting path.
Due to the great hydraulic flow passing through the pressure relief valve, there may be extreme heating of the pressure medium. It is very possible for the pump to be destroyed already after a relatively short time.
It is, of course, possible in theory to design the servo valve or other consumer system connected to the pump in such a way that, in all possible operating states, hydraulic medium can flow back from the delivery line via the servo valve or the consumer to the reservoir in order to ensure that an appropriate quantity of fresh and cooled hydraulic medium must always be supplied to the pump from the reservoir via the intake line. As a result, an adequate cooling of the pump could be theoretically ensured. Practically speaking, however, with such a design of the hydraulic system, a pump with increased capacity must be provided in order to be able to achieve a predetermined maximum pressure differential between intake side and delivery side of the pump. In other words, an increased drive power is required for the pump. This may cause problems in the case of motor vehicles because the vehicle engine can only deliver limited power in idling operation, at least if the idling speed is to be kept as low as possible.
In addition, in principle it would be possible to let the connecting line controlled by the pressure relief valve open out into the reservoir. Then, the hot hydraulic medium coming from the pressure relief valve could mix with the distinctly cooler hydraulic medium in the reservoir and correspondingly cooler hydraulic medium would be fed to the intake side of the pump. In this case, however, the connecting line would have to be relatively long if the reservoir is arranged some distance away. In addition, an additional connecting would have to be provided on the reservoir. If, finally, the pressure relief valve and the flow controller are to be accommodated together with the pump in a common block or housing to achieve as compact a design as possible, an additional connection for the connecting line would also have to be provided on this block as well.
The problems pointed out above have not been solved satisfactorily so far. For example, German Patent Specification 3,303,492 describes a special configuration of a flow controller in order to prevent the occurrence of flutter of the flow control piston.
German Offenlegungsschrift 3,822,970 discloses shock absorbers on which throttle valves controlling the damping resistance are provided with shape memory alloy springs whose elastic stress increases greatly at relatively high temperature and consequently increases the throttling resistance of the aforementioned valves. In this way, the damping force remains approximately constant even if the hydraulic medium in the shock absorber is heated considerably, and consequently becomes increasingly fluid.
Germany Offenlegungsschrift 3,313,666 shows a pressure control valve which is urged in the closing direction by a first spring. An additional bimetal spring opposes the bias of the first spring. This counteracting force can be controlled by heat acting on the bimetal spring. In this way, the closing pressure of the pressure control valve can be controlled externally.
Finally, in German Auslegesschrift 1,132,400 a thermostatically controlled valve is provided and has a closing body urged in the closing direction by a plurality of series-arranged different, bimetal springs. The closing pressure can thus be increased with increasing temperature.
An object of the present invention is to provide an apparatus for closed-loop or open-loop hydraulic flow control which, with a simple design, prevents an excessive temperature increase of the hydraulic medium in the pump even when the volume flow taken from the consumer system drops drastically or ceases altogether.
This object has been achieved according to the present invention with an apparatus in which the connecting line branches off downstream of the throttle and the flow control piston and/or the shut-off member of the pressure relief valve are actuated by a spring consisting of a shape memory alloy and having a pronouncedly temperature-dependent actuating force, such that the connecting path controlled by the flow control piston and the connecting line controlled by the pressure relief valve are released if the hydraulic medium exceeds a temperature threshold value.
The present invention is based on the general concept of controlling the actuating forces acting on the flow control piston and the pressure relief valve temperature-dependently by shape-memory alloy springs in order to shift the flow control piston completely into its open position if the temperature threshold value is reached. Consequently, the pump is relieved of virtually all the throttling resistances opposing the pump work, so that a further temperature increase of the hydraulic medium is reliably prevented.
Moreover, the present invention utilizes the discovery that, due to the arrangement of the branching of the connecting line downstream of the throttle, the heat of the hydraulic medium coming from the pump, which may be heated substantially, can heat a shape-memory alloy spring acting on the flow control piston and a shape-memory alloy spring of a shape memory alloy actuating the shut-off member of the pressure relief valve and consequently change the spring force significantly. In both instances, the connecting path controlled by the flow control piston is then released, and greater quantities of the hydraulic medium without excessive heating are able to flow through the path.
A particular advantage of the present invention is that the configuration of conventional apparatuses for open-loop of closed-loop hydraulic flow control only has to be modified slightly. Essentially, only at least one spring of a shape memory alloy need be additionally arranged in heat-conducting contact with the hydraulic medium.
According to a presently preferred embodiment of the invention, the shape-memory alloy spring has a greatly dropping or altogether ceasing actuating force below the temperature threshold value and a greatly increasing actuating force above the temperature threshold value, and actuates the flow control piston or the pressure relief valve respectively in the opening direction. As a result, an active opening force is exerted on the flow control piston or the pressure relief valve with increasing temperature of the hydraulic medium.
A redundant arrangement is also possible with the present invention, in that both the flow control piston and the pressure relief valve each interact with a spring of a shape memory alloy. Consequently a particularly high reliability against an excessive heating of the hydraulic medium in the pump is achieved.